In this study we examine the relative effect of abnormal tissue properties and the type and severity of lung destruction (emphysema) upon the pressure-volume characteristics of human lung. A loss in elastic recoil is pictured as following: 1) an increase in resting tissue length, 2) a uniform loss of elastic elements (panlobular emphysema), or 3) a localized loss of elastic elements as in centrilobular emphysema. These different mechanisms may act singly or in concert but the effect of these conditions should differ on the measured tissue properties, structure, and pressure-volume characteristics. The tissue properties of alveolar wall are estimated from tissue slips (25 x 25 x 150 microns). The maximum extensibility of alveolar wall (lambda m equals Lm/Lo) increases with age, probably due to an increase in resting tissue length (Lo) rather than maximum length (Lm). This measure, which is equivalent to the strain at which "yield" occurs, will be compared with structural changes (mean linear intercept, alveolar and ductal volume proportions of density, air-blood barrier thickness, etc.) in normal and emphysematous lung. From these data we can evaluate the relative part played by tissue properties apart from the loss in number and volume density of elastic elements (destruction) upon the measured pressure-volume characteristics. The type and severity of emphysema will be compared in their effect upon elastic recoil.